Difference between revisions of "Lizard Eye"

Latest revision as of 15:33, 5 January 2023

Introduction

Panther chameleon spectacle (© Tom Junek, Wikimedia Commons)

Common mydriatics have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Therefore, unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil mydriasis) in mammals are totally ineffective.

Another characteristic of the lizard eye is its ability to occupy both scotopic (light limited) and photopic (light rich) environments. Indeed, the morphology of the eye adapts and varies according to activity in these different light environments.

For information on examination of the eye, see Lizard Physical Examination.

Parietal Eye

The Parietal Eye of a Lizard (Wikimedia Commons)

Also Known As: Pineal Eye — Third Eye

A well developed parietal "eye" (with a rudimentary lens and retina) is found on the dorsal midline in some lizards, including the green iguana, blue-tongued skink and water dragon.

This sensory organ is connected to the central nervous system and the pineal gland by the small parietal nerve. It functions in hormone production (including reproduction) and thermoregulation (by acting as a light dosimeter). It detects both UV light and heat. Although sensitive to changes in light, it cannot form images. By detecting light and dark it allows lizards to detect the movement of predators. Sometimes referred to as "pineal eye" or "third eye", it it visible as an opalescent gray spot on the top of some lizard's heads.

Eyelids

They are usually present. The lower lid is movable, moving upwards to close the eye; in some lizards, this lid might be transparent, allowing vision even when the lids are closed. Lizard also have a well developed, semi-transparent, highly mobile third eyelid which is called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open.

In some chameleon species as well as some geckos and the oscellated skink (Ablepharus sp.) the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications.

Pupil

The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species. Geckos, such as the Tokay gecko, have a serrated pupillary opening resembling a series of small holes when the pupil is completely closed. This specificity allows for acute vision even in dim light.

Behaviour

When threatened or molested by other animals, such as dogs, coyotes or foxes, certain horned lizards (Phrynosoma cornutum, P. coronatum and P. solare) will respond by squirting a spectacular amount of blood from their eyes. They accomplish this by constricting venous outflow from the head which increases blood pressure and causes leakage from the ocular venous sinuses.

For more information on behaviour, see Lizard Behaviour.

Lizard Eye Learning Resources
Flashcards Test your knowledge using flashcard type questions	Reptiles and Amphibians Q&A 16

References

Engbretson, G.A. & Lent, C.M. (1976). Parietal eye of the lizard: neuronal photoresponses and feedback from the pineal gland. Proceedings of the National Academy of Sciences 73, 654-657
Fowkes, R (2008) Endocrinology of Special Species BVetMed Integrated Course RVC
Frye, FL & Williams, DL (1995) Self-Assessment Colour Review - Reptiles & Amphibians Manson
Girling, S.J. (2004). BSAVA Manual of Reptiles. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757
Hall, M.I. (2008). Comparative analysis of the size and shape of the lizard eye. Zoology 111, 62-75
Mader, D.R. (2005). Reptile Medicine and Surgery. Saunders. pp. 1264. ISBN 072169327X

Webinars

Failed to load RSS feed from https://www.thewebinarvet.com/ophthalmology/webinars/feed: Error parsing XML for RSS

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-{{unfinished}}
-[[Image:Eye_lizards.jpg|200px|thumb|right|'''Iguana eye (left), Tokay gecko eye (right)''' (© RVC and its licensors, Sean Bobbit, Sue Evans, Andrew Devare and Claire Moore. All rights reserved)]]
 ==Introduction==
+[[Image:Panther_Chameleon.jpg|300px|thumb|right|'''Panther chameleon spectacle''' (© Tom Junek, Wikimedia Commons)]]
+[[Image:Lizard_skull_mandible.jpg|150px|thumb|right|'''Parietal eye on the dorsum of the head''' (Copyright © RVC)]]
+Common [[Mydriatic|mydriatics]] have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Therefore, unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil [[Mydriatic|mydriasis]]) in mammals are totally ineffective.
-Common mydriatics have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Therefore, unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil (mydriasis) in mammals are totally ineffective.
+Another characteristic of the lizard eye is its ability to occupy both scotopic (light limited) and photopic (light rich) environments. Indeed, the morphology of the eye adapts and varies according to activity in these different light environments.
-Another characteristic of the lizard eye is its ability to occupy both scotopic (light limited) and photopic (light rich) environments. Indeed, the morphology of the eye adapts and varies according to activity in these different light environments.
+* '''For information on examination of the eye, see''' [[Lizard Physical Examination|Lizard Physical Examination]].
 ==Parietal Eye==
+[[File:Anolis carolinensis parietal eye.JPG|180px|thumb|right|'''The Parietal Eye of a Lizard (Wikimedia Commons)]]
+Also Known As: '''''Pineal Eye''''' — '''''Third Eye'''''
+A well developed parietal "eye" (with a rudimentary lens and retina) is found on the '''dorsal midline''' in some lizards, including the [[Green Iguana|green iguana]], [[Stump-tailed Skink|blue-tongued skink]] and [[Water Dragon|water dragon]].
-A well developed parietal "eye" (with a rudimentary lens and retina) is found on the dorsel midline in some lizards, including the green iguana, blue-tongue skink and water dragon. This sensory organ, connected to the central nervous system and the pineal gland by the small parietal nerve, functions in hormone production (including reproduction) and thermoregulation. Although sensitive to changes in light, it cannot form images. Sometimes referred to as "pineal eye" or "third eye", it it visible as an opalescent gray spot on the top of some lizard's heads.
+This sensory organ is connected to the central [[Lizard Nervous System|nervous system]] and the pineal gland by the small parietal nerve. It functions in [[Lizard Endocrine System|hormone production]] (including [[Lizard Reproduction|reproduction]]) and thermoregulation (by acting as a light dosimeter). It detects both UV light and heat. Although sensitive to changes in light, it cannot form images. By detecting '''light and dark''' it allows lizards to detect the '''movement of  predators'''. Sometimes referred to as "pineal eye" or "third eye", it it visible as an opalescent gray spot on the top of some lizard's heads.
 ==Eyelids==
-They are usually present although some geckos and the oscellated skink (''Ablepharus sp.'')have snake-like spectacles.
+They are usually present. The lower lid is movable, moving upwards to close the eye; in some lizards, this lid might be transparent, allowing vision even when the lids are closed. Lizard also have a well developed, semi-transparent, highly mobile third eyelid which is called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open.
-In some chameleon species, the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications. Lizard species lacking a spectacle have a well developed, semi-transparent, highly mobile third eyelid which is also called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open.
+In some [[Chameleon|chameleon]] species as well as some geckos and the oscellated skink (''Ablepharus sp.'') the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications.
-==The pupil==
+==Pupil==
-The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species. Geckos, such as the Tokay gecko, have a serrated pupillary opening resembling a series of small holes when the pupil is completely closed. This specificity allows for acute vision even in dim light.
+The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species. Geckos, such as the [[Tokay Gecko|Tokay gecko]], have a serrated pupillary opening resembling a series of small holes when the pupil is completely closed. This specificity allows for acute vision even in dim light.
-==Ophthalmologic Examination==
+==Behaviour==
-The eyes are usually considered as a barometer of general health and environmental conditions; therefore a full ophthalmologic examination should be performed in all cases.
+When threatened or molested by other animals, such as dogs, coyotes or foxes, certain horned lizards (''Phrynosoma cornutum, P. coronatum and P. solare'') will respond by squirting a spectacular amount of blood from their eyes. They accomplish this by constricting venous outflow from the head which increases blood pressure and causes leakage from the ocular venous sinuses.
-* Vision can and should be evaluated by observing certain behaviours such as a menace response.
+* '''For more information on behaviour, see''' [[Lizard Behaviour|Lizard Behaviour]].
-* Following a general assessment of the eye and associated structures (eyelids, tear film, globe), a more detailed examination should be performed using a form of focal illumination and a magnification system.
-* Examination of the ocular fundus to assess the retina (fundoscopy) can be carried out using a direct or indirect ophthalmoscope.
-* Mydriasis can be attempted either through general anaesthesia or intracameral injection of neuromuscular blocking agents such as curare or d-tubocurarine. These agents can also be applied topically; however their effectiveness may be affected by the variable corneal penetration of the drugs.
-* Other diagnostic tools, such as tonometry (measurement of the intraocular pressure), stains, cytology, bacteriology, histopathology and electron microscopy, in addition to routine diagnostic tools (haemotology, biochemistry, radiology and ultrasound) can also be used to detect an ocular disease or underlying problem in lizards.
+{{Learning
+|flashcards = [[Reptiles and Amphibians Q&A 16]]
+}}
 ==References==
 * Engbretson, G.A. & Lent, C.M. (1976). Parietal eye of the lizard: neuronal photoresponses and feedback from the pineal gland. Proceedings of the National Academy of Sciences 73, 654-657
+* Fowkes, R (2008) '''Endocrinology of Special Species''' BVetMed Integrated Course ''RVC''
+* Frye, FL & Williams, DL (1995) '''Self-Assessment Colour Review - Reptiles & Amphibians''' ''Manson''
 * Girling, S.J. (2004). ''BSAVA Manual of Reptiles''. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757
 * Hall, M.I. (2008). Comparative analysis of the size and shape of the lizard eye. Zoology 111, 62-75
 * Mader, D.R. (2005). ''Reptile Medicine and Surgery''. Saunders. pp. 1264. ISBN 072169327X
+{{review}}
+==Webinars==
+<rss max="10" highlight="none">https://www.thewebinarvet.com/ophthalmology/webinars/feed</rss>
+[[Category:Lizard Anatomy|E]]
+[[Category:Expert Review - Exotics]]